1. Field of the Invention
The present invention relates to a technique of reducing color fringing of a color shot image.
2. Description of the Related Art
In a color image capturing system, a color which does not originally exist appears as color fringing around a bright portion on an image owing to chromatic aberration of an imaging optical system. Color fringing readily occurs at a portion apart from the center wavelength of the imaging optical system. In a visible light color image capturing system, an artifact in blue, red, or purple as a mixture of blue and red appears as a fringe. This artifact is called color fringing or purple fringing.
Chromatic aberration can be optically suppressed to a certain degree by combining lenses having different dispersions.
These days, as digital cameras are becoming compact, demands arise for increasing the resolution of the image sensor and downsizing the optical system. It becomes difficult to satisfactorily suppress chromatic aberration by only the optical system. Artifacts need to be reduced by image processing.
Chromatic aberrations are roughly classified into transverse chromatic aberration (chromatic aberration of magnification) and longitudinal chromatic aberration (on-axis chromatic aberration). Transverse chromatic aberration is a phenomenon that the image location shifts in a direction along the image plane depending on the wavelength, as shown in FIG. 1. Longitudinal chromatic aberration is a phenomenon that the image location shifts in a direction along the optical axis depending on the wavelength, as shown in FIG. 2.
A digital image capturing system for the primary color system can correct transverse chromatic aberration by geometric transform of adding different distortions to the R (Red), G (Green), and B (Blue) color planes, as disclosed in U.S. Pat. No. 6,724,702B1.
As for longitudinal chromatic aberration, for example, an image which is in focus on the G (Green) plane serving as the center wavelength of the visible light region blurs on the R (Red) and B (Blue) planes serving as the ends of the visible light region. Longitudinal chromatic aberration cannot be corrected by geometric transform, unlike transverse chromatic aberration. Hence, there is proposed a method of correcting longitudinal chromatic aberration by adding different edge enhancement processes to the R, G, and B planes, as disclosed in Japanese Patent Laid-Open No. 2003-018407. Further, there is proposed a method of making longitudinal chromatic aberration less conspicuous by decreasing chroma in a region where color fringing occurs, as disclosed in Japanese Patent Laid-Open No. 2001-145117.
However, deconvolution and approximate edge enhancement processing as described in Japanese Patent Laid-Open No. 2003-018407 cannot obtain a proper result unless an accurate point spread function is known. In an image capturing apparatus such as a camera in which the object distance and shooting conditions change, the state of the optical system including the zoom position, F-number, and focus position also changes. It is difficult to obtain an accurate point spread function. Deconvolution can be used in only the linear reaction boundary of the image sensor, and cannot reduce color fringing around saturated pixels.
The optical system of a general color image capturing apparatus optically corrects chromatic aberration to a certain degree, and color fringing hardly stands out in a normal brightness range. Instead, when an excessively bright object exists within the frame to saturate pixels, a small quantity of leakage light which cannot be completely corrected often acts as a significant quantity and causes color fringing. That is, the technique described in Japanese Patent Laid-Open No. 2003-018407 cannot correct this color fringing.
As described in Japanese Patent Laid-Open No. 2001-145117, the process to decrease chroma can cancel a fringing color and reduce unnaturalness. However, the original object color is also influenced by this process and becomes grayish regardless of the presence/absence of color fringing.